Without limiting the scope of the disclosure, this background is described in connection with external fixation devices and specifically connection struts and rods. Generally, external fixation devices are commonly used in a variety of surgical procedures including limb lengthening, deformity correction, fracture reduction, and treatment of non-unions, mal-unions, and bone defects. The process involves a rigid framework comprising several rings that are placed externally around the limb and attached to bone segments using wires and half pins, which are inserted into the bone segments and connected to the related section of the external rigid framework. The opposite rings of the rigid framework are interconnected by either threaded or telescopic rods directly or in conjunction with uni-planar or multi-planar hinges, which allow the surgeon to connect opposite rings that are not parallel to each other after manipulation with bone segments either rapidly (acutely) or gradually over a period of time.
For example, in bone fracture reduction or non-union treatment, the wires and half pins are inserted into each bone segment and attached to rings of a rigid framework. The rigid framework is used to acutely reduce a displacement and restore alignment between the bone segments. During the realignment of the bone segments, the orientations of opposite rings are often not parallel. Those opposite rings of the rigid framework are connected together by threaded or telescopic rods with attached uni-planar or multi-planar hinges. This allows the opposite bone segment to be rigidly fixed until complete fracture healing or bone consolidation is completed.
Also for example, in limb lengthening, the bone is surgically divided into two segments and wires and half pins are inserted into bone segments above and below the surgical bone cut and attached to rings of a rigid framework interconnected by struts or telescopic connection rods. The rigid framework is used to gradually push the two bone segments apart longitudinally over a period of time (e.g., one millimeter a day), which allows the bone to gradually form in the gap between bone segments created by this distraction technique. Once the desired amount of lengthening is achieved (e.g., 5-6 cm), the external apparatus is stabilized into a fixed position and left on the bone segments until mineralization of the newly formed bone is complete (e.g., 3-6 months, depending on the nature of pathology and amount of lengthening).
Similarly, in deformity correction, the bone is surgically divided (usually at the apex of the deformity) into two segments, and wires and half pins are inserted into bone segments above and below the surgical bone cut and attached to rings of a rigid framework. Opposite rings of the rigid framework are connected together by threaded rods with attached uni-planar or multi-planar hinges and an angular distractor is used to gradually push the two bone segments apart angularly over a period of time.
For various bone treatments, introducing controlled destabilization can accelerate bone healing and significantly improve the strength of the fracture callus. Gradually increasing a load is an important part of the bone healing process. To achieve such controlled destabilization, the external fixation devices can be dynamized or minimized. There are many ways of achieving dynamization, examples including, for a unilateral fixator, removing its bars, sliding the bars further away from the bone, removing its pins, and/or releasing tension or compression from the system, and for a circular frame, removing its wires, releasing tension from the wires, removing its connection rods between rings, removing the rings from a ring block, and/or releasing tension or compression from the system. These techniques can be problematic since they often result in wide variations in the level of instability and may not effectively limit the dynamization to a desired direction or axis of movement.
One common fixation device is a circular metal structure known as the Ilizarov Apparatus. The Ilizarov Apparatus, when used for limb lengthening or deformity correction, consists of several rings or arches that are placed externally around the limb and attached with wires and half pins to surgically separated bone segments. For limb lengthening, the opposite rings are interconnected directly by three or four threaded or telescopic rods that are regularly adjusted in length for gradual separation of bone segments longitudinally. For angular deformity correction, the opposite rings of the Ilizarov apparatus are connected by a pair of hinges that provide an axis of rotation for the bone segments and an angular distractor that gradually pushes apart two rings and associated bone segments.
Another common external fixation device is the Taylor Spatial Frame, which is a hexapod type external fixation device based on a Stewart platform but shares many components and features of the Ilizarov Apparatus. The Taylor Spatial Frame consists of two external fixation rings attached to bone segments by wires and half pins and connected together by six telescopic struts with multi-planar hinges located at both ends of the strut. Each strut may be lengthened or shortened as necessary to either pull two interconnected ring segments towards each other or push them apart. Adjustment of strut length allows manipulation of the bone segments acutely or gradually in six axes (e.g., lengthening/shortening external/internal rotation, anterior/posterior horizontal translation, medial/lateral horizontal translation, anterior/posterior angular translation, and medial/lateral angular translation) to perform limb lengthening and correct angular, translation and rotational deformities simultaneously.
The Taylor Spatial Frame includes a plurality of struts interconnecting a pair of rings. The wires, half pins, struts and other connection and assembly elements of the frame are connected to the rings via apertures. All of the apertures or holes are located on the same ring surfaces extending from the upper ring surface to the lower ring surface. This creates a positioning problem for wire and half pin attachment and placement of additional connection rods and assembly elements due to the competition for apertures in the fixation ring of the frame or the wires and pins interfering with the connections of the struts to the apertures.
Each strut of the Taylor Spatial Frame has a threaded rod partially disposed inside of a hollow shaft, and the hollow shaft includes an adjustment nut that mates with the threaded rod. To effect either a coarse adjustment (rapid strut length adjustment) or a fine adjustment (gradual strut length adjustment) to the length of the strut, the same threaded rod is pulled out or pushed in relation to the hollow shaft. Because the threaded rod has a finite length, however, using the same threaded rod for rapid strut length adjustment limits the total length of threaded rod available for gradual strut length adjustment during, for example, limb lengthening and deformity correction. As a result, a time consuming exchange of the struts for longer ones during the treatment is required.
Additionally, the replacement or removal of a strut from the Taylor Spatial Frame during the course of treatment is impossible without using external support or other stabilization mechanism to support the rest of frame. The struts of the Taylor Spatial Frame must be connected at the top or bottom of the rings. Such connections require the use of either ball joints in the apertures of the rings or universal joints extending from the top or bottom surfaces of the rings. The Taylor Spatial Frame, however, does not include any locking mechanism for temporarily locking the universal joints or ball joints in their orientation. As a result, if one strut is removed from the frame, it would become unstable and collapse.